1. Field of the Invention
This invention relates to a silicate fluorescent substance to be employed in a display device, an illuminator or various light sources, and to a light-emitting device employing the silicate fluorescent substance.
2. Description of the Related Art
A light-emitting diode (hereinafter referred to as LED) which is composed of a combination of a light-emitting element as an excitation light source and a fluorescent substance is well known. In this LED, it is possible, through variously changing the combination of the light-emitting element and the fluorescent substance, to realize various luminescent colors. Among them in particular, in order to realize a light-emitting device which is capable of emitting white light or so-called white LED, there have been proposed various methods such as a method wherein a light-emitting element which is capable of mainly emitting blue light and a yellow fluorescent substance are employed as a combination, and a method wherein a light-emitting element which is capable of emitting near-ultraviolet ray, a blue fluorescent substance, a yellow fluorescent substance and a red fluorescent substance are employed as a combination. As for the yellow fluorescent substance to be employed in the white LED, although a YAG fluorescent substance is well known, since the YAG fluorescent substance is weak in luminescence when it is excited with light having a wavelength ranging from 360 nm to 410 nm, the employment thereof is limited to an LED where a blue light source is employed.
As for the yellow fluorescent substance which is capable of emitting yellow light as an emission spectrum when it is excited with light having a wavelength ranging from 360 nm to 500 nm, there is known a silicate fluorescent substance having a composition represented by: M2SiO4:Eu. Among them, although one having a composition of: Ba2SiO4:Eu2+ is limited in crystal structure to orthorhombic system, one having a composition of: Sr2SiO4:Eu2+ is reported to change its crystal structure from monoclinic system at a temperature of not more than 85° C. to orthorhombic system at a temperature higher than 85° C.
The luminescence of the orthorhombic system of Ba2SiO4:Eu2+ and of the monoclinic system of Sr2SiO4:Eu2+ would be greenish yellow having a peak wavelength of 520-540 nm or so. When a portion of Sr of Sr2SiO4:Eu2+ is replaced by Ba, it would become possible to obtain the orthorhombic system even at room temperature and the luminescence thereof would become yellow of high color purity having a peak wavelength of 570 nm. Therefore, in order to obtain yellow luminescence of high color purity in a silicate fluorescent substance represented by a composition of M2SiO4:Eu, it is indispensable to include Ba. Further, in order to obtain an emission band having a desirable wavelength within the range of 520 to 600 nm, it is required to adjust the composition of fluorescent substance through the control of the content of Ba.
However, since a Ba compound has a bad influence on human body, the employment thereof is restricted by the toxic substance control regulations. Therefore, even if the fluorescent substance containing Ba compound is capable of emitting yellow luminescence of high color purity, the content of Ba compound should preferably be limited as minimal as possible.